Various types of semiconductor devices are manufactured in much the same way. A starting substrate, usually a thin wafer of silicon or gallium arsenide, is masked, etched, and doped through several process steps, the steps depending on the type of devices being manufactured. This process yields a number of die on each wafer produced. The die are separated with a wafer saw, and then packaged into individual components.
During the packaging process, several semiconductor die are attached to a lead frame, often with a material such as epoxy or other viscous adhesives. Bond wires couple each of several bond pads on each die to conductive lead "fingers" on the lead frame. Leads are interposed between the lead fingers and the host. The die, the wires, and a portion of the leads are encapsulated in plastic. The leads on the lead frame couple the die with the device into which the component is installed, thereby forming an electrical pathway and a means of input/output (I/O) between the die and the host.
One step of semiconductor manufacture that is not without problems is the die-lead frame attachment. FIG. 1 shows a die 10 attached to a paddle 12 of a lead frame 14. The paddle 12 is usually about the same length and width as the die 10, but is shown larger than the die 10 for illustration purposes. Die pads 16 are attached by bond wires 18 to lead fingers 20 of the lead frame 14. The die 10 and a portion of the lead frame, shown by 22, is then encapsulated in a protective plastic casing. The lead portion 23 of the lead frame 14 will extend to the outside of the plastic, thereby allowing a means of I/O between the die 10 in the package and the host into which the package is installed. Below is a partial list of current methods of attaching the die 10 to the paddle 12:
1. Epoxy Paste--The epoxy is dispensed on the die paddle area of the lead frame. The die is lowered into the uncured epoxy by a surface contact tool or an edge contact only tool (collet) and held by the tool long enough to ensure adhesion. X-Y movement (scrub) is sometimes used to increase adhesion and speed the process. This process requires a follow-on cure in a separate cure oven.
2. Epoxy Film--An epoxy film is dispensed on the die paddle of the lead frame and the die is lowered down to the film surface. Bonding is aided by pressure between the die and paddle. This process requires a follow-on cure in a separate cure oven.
3. Epoxy Film on Tape--An epoxy film is applied to both sides of a supporting tape, and the tape is interposed between the die and the paddle. Pressure is applied between the die and paddle to improve bonding, then the assembly is cured in a separate curing step.
4. Eutectic--A metal with a low melting temperature (solder) is dispensed onto the lead frame paddle. A die is placed on the dispensed metal. Adhesion is obtained by an intermixing of the die backside and the metal. Controlled pressure, scrub, and temperature are often used. No follow-on cure is required.
5. Soft Solder--Same process as in Eutectic except that the metal does not mix with the backside material.
6. Glue--A conductive or non-conductive glue can be used as required. The glue is normally a quick set type adhesive which requires no later cure.
Various problems are associated with the connection of the die to the die paddle, and with the connection of the wires from the die pad to the lead fingers. A few of the difficulties associated with the die-lead frame attachments and wire bonds are described below.
1. Lead movement--Lead movement occurs after wire bonding. The lead fingers are relatively long for their thickness, and therefore can bend and move around quite easily. As the assembly is transported to location of the encapsulation step, the wire connections are easily broken. Lead movement increases as the thickness of the lead frame decreases.
2. Paddle--The paddle of the lead frame itself is stamped to a lower plane during the manufacturing process, thereby positioning the bottom of the die below the lead fingers on the lead frame, as shown in FIG. 2. The paddle downset 24 positions the die 10 so that the distance from the top of the die 10 to the bottom of the paddle 12 is centered with respect to the lead frame 14. For example, in FIG. 2, if an 11 mil die 10 is attached with a 1 mil thick attach material 26 to a 10 mil lead frame 14 having a paddle downset 24 of 6 mils. In the configuration of FIG. 2, the die 10 extends 6 mils above the lead frame 14, with 6 mils of paddle 12 extending below the lead frame 14. In this configuration, the amount of material above the die 10 equals the amount of material below the lead frame 14, and therefore the flow of plastic during the encapsulation process would not unduly stress the bond wires and would produce a higher yield.
In contrast, FIG. 3 shows a 11 mil thick die 10 attached with a 1 mil thick attach material 26 to a 10 mil thick lead frame 14 without a paddle downset. The top surface of the die 10 extends 12 mils above the lead frame 14, with nothing extending below the bottom of the lead frame 14. This configuration would produce a low yield, due to the unfavorable flow of plastic material around the die 10 and bond wires 18 during the encapsulation step. While some leeway is allowable, a 12 mil difference is not. In addition to problems caused by the uneven flow of encapsulation material, a package without a paddle downset bows due to the shrinkage of encapsulation material as it cures.
In addition to providing a centered die, the downset allows for shorter bond wires. Longer bond wires have a higher inductance than shorter bond wires, and also have a higher probability of wire sweep during encapsulation. Other problems known to artisans skilled in the art can also occur from bond wires which are overly lengthy.
As packages become thinner, it becomes necessary to make the die-lead frame assembly thinner. One way this is currently accomplished is to produce a thinner lead frame. FIG. 4 shows a 5 mil thick lead frame 40 supporting the 11 mil thick die 10 of FIGS. 2 and 3. This configuration produces a 17 mil thick assembly. One problem is that a 5 mil thick lead frame 40 is very flimsy, and further compounds the problems of lead movement listed above. It also produces package leads which can easily be damaged or misaligned.
Having a paddle downset also creates problems, as a lead frame with a paddle downset is not as manufacturable as a lead frame without a paddle downset. The paddle downset requires specialized fixtures which are not necessary for lead frames without the downset.
The paddle itself also creates extra expense. The die paddle is typically gold or silver plated along with the conductive leads, in part because the paddle cannot economically be masked during plating of the conductive leads. The leads are plated to provide the proper metallic surface to which to wire bond since the bond wire will not stick directly to the material usually used for the lead frame, such as copper or alloys. The plating of the paddle, however, serves no functional purpose. This unnecessary gold or silver plating of the paddle, which is a relatively large surface, adds unnecessary cost to the product.
3. Corner crack--Occasionally a corner of the die will break, thereby making the semiconductor useless. This can result from an uneven coefficient of expansion between the die and the die paddle. After the die is attached to the lead frame, the assembly is heated at the wire bond step to attach the wire to the die pad. If the die and the paddle expand at different rates, the corner of the die may crack. Corner crack can also occur from stress on the die due to shrinkage of the encapsulant epoxy as it cures, although in recent years chemical improvements in encapsulant epoxy has reduced this cause of corner crack.
Another cause of corner crack is metal buses, such as power or ground buses, which are often placed around the outer edges of the die during design. The buses, which are relatively large compared to the rest of the circuitry on the die, can cause the corners of the die to crack from thermal mismatch between the metal of the die and the substrate material.